If you have any passengers travelling on the Visa Waiver Program into Canada, they will have a few extra steps before being allowed to enter Canada. Only U.S. nationals are exempt from this requirement. Let's say, for example, you have a citizen of the United Kingdom spending time in the United States on the Visa Waiver Program, also known as a "green card." This person will have to obtain Canadian electronic Travel Authorization (eTA).
This program will be instituted on 15 March 2016. I've provided an overview of the process below, but your best source of information is from Canada: http://www.cic.gc.ca/english/visit/eta.asp.
For more information about the other aspects of this:
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
Figure: "Do I Need an eTA?" from Canada eTA website.
As of March 15, 2016, Canada will require Electronic Travel Authorization (eTA) for all visa-exempt foreign nationals entering the country by air. Effective this date, visa-exempt foreign nationals, other than U.S. citizens, must have an eTA prior to being processed by customs, immigration, and quarantine (CIQ) and entering Canada. In the past visa-exempt foreign nationals could travel to Canada without any specific advance travel authorization. This changes on March 15, 2016, with the introduction of the new eTA requirements for visa-exempt foreign nationals, regardless of their age. Only U.S. nationals are exempt from this requirement. U.S. permanent residents (Green Card holders) require eTAs to arrive in Canada by air. For example, if you’re a visa exempt national of the UK or Japan you’re now required to have an eTA to enter Canada, even though you did not require any advance travel authorization in the past. Note that new eTA requirements also apply to tech stops in Canada, even if passengers and crew do not leave the aircraft.
The screen capture here shows the results for a person travelling with a passport issued by France. You can check the countries you are interested in here: http://www.cic.gc.ca/english/visit/visas.asp.
To apply for an eTA, see: http://www.cic.gc.ca/english/visit/eta-start.asp.
According to contacts at Universal Weather & Aviation, Inc., once your eTA has been approved there’s no particular paperwork you need to carry it with you. An eTA is electronically linked to your passport. Note that you must always carry the passport you used to apply for the eTA with you.
According to contacts at Universal Weather & Aviation, Inc., if you arrive in Canada without a required eTA you may apply for one, via any computer or mobile device, prior to clearing the CIQ process. But, you will not be processed by CIQ until your eTA has been approved and confirmed.